1. Field of the Invention
This invention relates to a thermal printing head for a thermal printer and to an anti-abrasion layer formed as an uppermost or outer layer for protecting heating elements and electrodes from abrasion type wear during contact with print paper or an ink ribbon. More specifically, this invention relates to improvement in the anti-abrasion layer adaptable for high speed printing.
2. Description of the Prior Art
Non-impact thermal printers have the beneficial features of silent, relatively high speed and high dot density printing operation. These type printers also allow a compact and low cost design as compared to other non-impact printers employing laser or ink jet technologies.
Letters or graphic patterns are formed as black or colored dots developed on thermosensitive paper or ordinary paper, as illustrated in prior art FIG. 1. When printing paper 1 is fed between thermal printing head 2 and platen 3, fine or very small heating elements 4, disposed on a substrate 5 and arranged horizontally in a line, are selectively supplied with electric current usually in the form of pulsed signals. These elements 4 heat the paper 1 or ink ribbon (not shown). As a result, a specified number of black or colored dots in a horizontal line are generated on the paper 1. Thus, letters (for example, A and B in FIG. 1) or graphic patterns are created as the paper is fed.
FIG. 2 is a cross-sectional view of a thermal printing head, including an insulating substrate 5 such as an alumina (Al.sub.2 O.sub.3) ceramic and a temperature insulating glaze layer 6, formed on the substrate for preventing heat loss through the substrate 5. A heating element layer 7 is formed on the glaze 6, usually as a thin film of a material such as a tantalum nitride (Ta.sub.2 N), and conductors 8, 9 and 9' are formed on the heating element layer 7, except at the specified portions (see R and R" in FIG. 2), to supply the portions R and R' with electric power. An anti-oxidation layer 10 for protecting the heating element layer 7 from oxidation is formed on the conductors 8 and 9 and the exposed portion of heating layer 7, and an anti-abrasion layer 11 for protecting the heating elements 4 and conductors 8 and 9 from abrasion caused by friction with the print paper or the ink ribbon (i.e. a thermal transfer ink ribbon) is formed on the anti-oxidation layer. Electrodes 12 are transversely formed on the electrode 8 with the interposition of insulating layer 13 therebetween, and specified electrodes 12 are connected to corresponding conductors 8 via a through-hole. A diode 14 is used as a gate device for the electric current supplied to the heating element 4, and is formed between electrodes 9 and 9'.
FIG. 3 is an enlarged perspective view of heating elements 4 and electrodes 8 and 9, which are disposed side by side in a row on a substrate (not shown). FIG. 4 is a cross-sectional view of FIG. 3 along the line X-Y in FIG. 3 where like reference numerals designate like or corresponding parts throughout. As illustrated by FIG. 4, the surface of anti-abrasion layer 11 is always subject to friction caused by the printing paper 1 as it is fed past the head, and thus causes abrasion of the layer 11. The anti-abrasion layer 11, in the prior art, is generally tantalum pentaoxide (Ta.sub.2 O.sub.5), because of its excellent abrasion resistance and ability to adhere to other materials of the printing head. However, Ta.sub.2 O.sub.5 does not protect the heating elements 4 from oxidation during normal operation. Therefore, it is necessary to provide an anti-oxidation layer 10 of SiO.sub.2, for example, between the heating element 4 and the anti-abrasion layer 11, when the heating elements 4 are made of a material such as Ta.sub.2 N, for example, whose oxidation resistance is relatively low.
Recent high speed operating requirements for thermal printers, require energization of the heating elements using narrow width electric pulses such as pulses of 1 millisecond (ms) in width, as compared with 2 to 3 ms in conventional thermal printers. Such high speed operation frequently causes cracks in the anti-abrasion layer 11. The cracks usually extend to the surface of the heating elements, even through the anti-oxidation layer 10 provided between the heating elements 4 and the anti-abrasion layer, thereby allowing the heating elements to be exposed to the air. As a result, the heating elements are oxidized as they are heated, and the actual operational life of a thermal printing head is shorter than the expected life. The life of a thermal printing head, as shortened by such cracks, is occasionally as short as one hundredth that associated with the life of the head when abrasive wearing is the failure mode.
In the prior art thermal printer technology, the occurrence of the cracks in the anti-abrasion layer 11 has been attributed to stress caused by thermal shock when the pulsed type electric power is applied to the heating elements 4. Therefore, improvements directed at preventing the cracks have been focused on providing an anti-abrasion layer 11 subject to reduced stress. Some proposed techniques are disclosed in Japanese patent applications: Tokukai-Shou Nos. 56-145072 to 56-154075, all published Nov. 28, 1982. The concept advanced by these disclosures is to form an anti-abrasion layer in which the chemical composition is not uniform across its thickness. For example, when an anti-abrasion layer is formed of Ta.sub.2 O.sub.5 and silicon dioxide (SiO.sub.2), Ta.sub.2 O.sub.5, which is a hard component, is richer in near a surface region, while SiO.sub.2, which is a soft component, is richer in or near an underlying region. Such a composition change is created using a continuous variable composition layer or discontinuous multiple layers each of which differs in composition. The thermal printing head described in the above references does not include an anti-oxidation layer and the variable ratio mixture anti-abrasion layer described therein also acts as an anti-oxidation layer. The references particularly state that an anti-oxidation layer is not needed.
The above-mentioned techniques require complicated process control and a special apparatus for fabricating the anti-abrasion layer, and thus when using these techniques it is hard to provide low cost thermal printing heads.